1. Field of the Invention
The present invention generally relates to a device and method of coupling elements that require precision alignment, and more particularly to a method of coupling optical components that require fine alignment to achieve efficient optical coupling.
2. Description of Related Art
The coupling of laser light into optical fiber is a critical technology for telecommunications, metrology and a wide variety of other applications. Efficient optical coupling typically requires diffraction-limited focusing of light from a source (usually a laser or LED) into the core of an optical fiber. Since the core of a typical optical fiber is less than 10 microns in diameter, the tightly focused light must be directed to the fiber core with great precision and this precise alignment must be maintained over extended periods (e.g. years) despite changes in temperature and humidity, mechanical vibrations and other hazardous conditions. The alignment becomes increasingly more critical if the single mode fibers have an extremely small diameter (e.g. about 3.0 microns) such as those used with shorter wavelength lasers (e.g. GaN-based lasers).
Using conventional assembly techniques, mechanical tolerance stack-up in commercial diode laser packages, standard machined parts and standard commercial optics amounts to about 1 mm3 of uncertainty. If, for example, a fiber coupling lens is mounted in front of a diode laser in a standard CD-ROM package then the position of the focus of light from the diode laser may be anywhere within a volume of about 1 mm on a side. Since a typical optical fiber for visible light has a core of just 4 microns in diameter, it is evident that a precision, sub-micron alignment is required within this much larger (approximately 1 mm3) volume. An analogous problem arises when attempting to couple light from a collimated laser such as a gas, ion or diode-pumped solid state (DPSS) laser.
The conflicting requirements of alignment range and tolerance have led to two basic classes of approaches. The first approach is to start with bare laser die and, using alignment techniques such as silicon optical bench methods, position the laser emission aperture and lensed fiber to the required tolerances, and fix the elements in place through laser welding or precision soldering. The second approach is to use an adhesive to fix components in place after the precision alignment step is finished.
The first approach requires a semiconductor-processing infrastructure (clean-room environment, manipulation under microscope, and ability to handle bare semiconductor die) and greatly restricts the versatility of the process for two reasons. First, this approach can only be economical for very large quantities and second, since only bare die compatible with the manufacturing process can be handled, there is no way to take advantage of the great variety of diode lasers of different wavelengths and powers now available in industry-standard, TO-18 CD-ROM packages. In addition, it is typical for misalignment to occur during the welding process due to uneven thermal expansion; this misalignment, know as post-welding shift, must be corrected by a subsequent re-bending step. Other techniques are subject to similar misalignment problems due to the fastening process.
The second approach (which uses an adhesive) depends wholly on the choice of adhesive, and the requirements on the adhesive are severe. This adhesive must be a low-viscosity liquid during the precision alignment so it does not interfere with the alignment. Furthermore, the adhesive must be chosen to cure rapidly after application but without shrinkage or excessive outgassing, which causes misalignment. It must not undergo chemical reactions or outgas upon exposure to laser light and it must maintain mechanical stability and adhesion for many years over a wide range of temperatures and humidity. Proprietary adhesives have been used to meet some of these requirements, but the perfect adhesive does not exist. Moreover, the introduction of adhesive compounds into an opto-mechanical system represents a long-term reliability risk.
As described above, these fiber-coupling methods align the optical elements and then fasten them together. Because the fastening occurs after alignment, the fastening process causes some misalignment that must be corrected after the alignment process has been completed. For example, mechanical shifting occurs after a weld due to the rapid heating and cooling of the metals being attached. As another example, slippage typically occurs when bolting two objects together. Unless a subsequent fine alignment process is available, this shifting or slippage will remain. Such fine alignment, especially in three dimensions, has been difficult to accomplish in practice. For example, the subsequent re-bending discussed with the first approach above is difficult and unpredictable.
In devising suitable aligning mechanisms and procedures, a complicating factor is that alignment must be achieved in three dimensions. There are many mechanisms for two-dimensional positioning, but adjusting the position along a third axis without losing alignment along the other two axes is very difficult. This problem is often attacked by breaking up the optical alignment process into a pre-focusing or pre-collimating step to fix one degree of freedom, followed by a final two-dimensional alignment step. This type of two-step approach can be effective, but increases process time and cost.
U.S. Pat. No. 5,351,330 to Jongewaard describes a fiber-coupled laser assembly in which lateral alignment is accomplished by moving the coupling optics, which are mounted on a flexure. The flexure allows fine positioning in two dimensions only. In addition, focusing must take place in a separate step after which the ferrule is set in place by a set screw, which prevents further fine adjustment.
U.S. Pat. No. 6,276,843 to Alcock et al. teaches the use of a kinematic positioner that allows a pre-collimated fiber/lens assembly to be positioned in front of a collimated laser beam. This positioner is adjustable but is bulky and contains many expensive components. Lateral adjustment is accomplished with screws positioned perpendicular to the axis of the barrel; therefore focus adjustment of the lens in front of the fiber (and collimation of the source laser) must be performed in a separate step.
U.S. Pat. Nos. 4,753,510 and 4,889,406 to Sezerman disclose a mechanism for coupling light from a fiber to a fiber or from a light source to a fiber comprising two pre-collimated sections, each containing a fiber or light source and a lens positioned to collimate light emerging from the fiber or light source. In one embodiment, the two sections are separated by a resilient member, such as an O-ring, and fastened together by a set of screws, some in tension and some in compression. Tilting of the two sections can allow lateral alignment, but it is difficult to tighten all of the screws properly without misaligning the mechanism. Without proper tightening of the screws, however, slippage occurs and the alignment degrades. The alignment stability of the completed mechanism is therefore highly dependent on the skill of the assembler. In addition, separate pre-focusing steps are required for both the source and receiver sections. Finally, the resilient member may introduce contamination problems.
In summary, there is a need for a low-cost, reliable, secure and adhesive-free device for coupling optical components. Such a device would preferably possess a large alignment range to accommodate mechanical tolerances in the optical elements and mounting parts, while achieving sub-micron resolution in the finished product. Furthermore, there is a need for a method of achieving efficient coupling that is capable of fine alignment in all three dimensions.